1. Field of the Invention
The invention relates to a phase-locked loop comprising a controllable oscillator which is phase-coupled through a phase detection arrangement and a lowpass filter to an input carrier applied to a signal input of the phase detection arrangement, and relates also to a directly mixing synchronous AM-receiver provided with an RF input which is coupled to a synchronous AM detector to which a synchronous RF carrier is applied through a synchronous in-phase carrier input for direct conversion of an RF-AM receiver signal to the base band, comprising such a phase-locked loop for generating the synchronous RF carrier.
2. Description of the Prior Art
A phase-locked loop of the type described, used in a directly mixing synchronous AM-receiver--alternatively called Costas receiver--is shown in, for example, German Patent Application No. 3240565 laid open to public inspection.
The said phase-locked loop is generally used for generating a carrier which is phase-synchronous with an input carrier applied to the signal input and is used in many types of arrangements. An important field of application beside the afore-mentioned directly mixing synchronous AM-receivers is formed by stereo decoders such as, for example, the integrated FM stereo decoder TEA 5580 in which the phase-locked loop is used for generating one or more carriers which are phase-synchronous with the 19 kHz stereo pilot signal of a stereo multiplex signal.
The accuracy with which the phase of the generated carrier follows the phase of an input carrier is limited in the known phase-locked loop by an undesired parasitic DC offset of the oscillator control signal. This parasitic DC offset is mainly caused by switching and/or bias voltage asymmetries which cannot be entirely avoided in practice. As a result the generated carrier exhibits a fundamental phase error with respect to the input carrier, which error disturbs the signal processing performed with the generated carrier. For example, such as phase error, when using the known phase-locked loop in a stero decoder, gives rise to undesired cross-talk between the left and/or right-hand stero channels.
When the known phase-locked loop is used in a directly mixing synchronous AM-receiver--such as the above mentioned Costas receiver--the said phase error causes a phase difference between the modulated RF input carrier applied to the synchronous detector and said generated carrier and hence a distortion of the picture and/or sound signal at the output of the synchronous detector. Dependent on the field of application of the receiver, phase errors are permissible to a limited extent: for example, in a well-tried construction of double sideband AM radio receiver it was revealed that the signal distortion at a phase error of up to approximately .pi./6 rad was still acceptable and at a further increase of the phase error its audibility was found to increase particularly strongly. When used in a TV receiver, the acceptable phase error is much smaller.
The amplitude of the phase error significantly increases when the S/N (signal-to-noise) ratio of the RF input carrier decreases, particularly due to a decrease in field strength. As a result the dynamic input range, or the dynamic rnage of the known directly mixing synchronous AM-receiver is comparatively small. In the given embodiment the ratio between the field strength at which the receiver was just not limiting and the field strength at which the said permissible phase error of .pi./6 rad occurred was found to be approximately 30 dB. The range of applications of the known directly mixing synchronous AM-receiver is therefore very limited and is much smaller than, for example, that of existing integrated AM synchronous receivers which are based on the superheterodyne principle and which may have a dynamic range of 60 to 70 dB.